[Phys-L] Re: thrust > drag

[Bob LaMontagne <rlamont@POSTOFFICE.PROVIDENCE.EDU>]

You have probably heard of the expression "that dog won't hunt". Well, "that
plane won't fly". The lift vector is not in the opposite direction to the
weight during straight and level flight. It is tilted somewhat to the rear
of the aircraft. If the wing does not have an effective angle of attack,
there is no lift generated. Horizontally, you have thrust forward, with drag
and a horizontal component of lift acting backward. Therefore, the thrust is
always greater than the drag.

I hate well meaning but incorrect questions. If a student has a little more
knowledge than the questioner, the student loses credit.

Bob at PC

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
Behalf Of Chuck Britton
Sent: Friday, July 01, 2005 11:28 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: thrust > drag

I neglected to say that the question is accompanied by an
illustration with four orthogonal directions clearly labeled weight,
lift,(vertically directed), drag and thrust (horizontally directed).
Also the velocity is clearly stated as being in the horizontal
direction.

Our use of these terms in intro physics is indeed greatly simplified.
(but useful IMHO)

Most students state that the weight and lift forces are equal because
there is no acceleration in the vertical direction. (I take this as
being a correct answer.)

Far too many students will state on a final assessment that the
thrust must be greater than the drag because the airplane is moving
forward. (I consider this to be an incorrect response)

I don't follow all of the detailed  'explanations'   that are laid out
below.

a) is said to be the correct response but further elucidation states
that thrust is a bit larger than drag.
        typo somewhere here?

We'll get into the Navier-Stokes equations in a later course.


At 10:47 PM -0400 7/1/05, John Denker wrote:
> On 07/01/05 16:04, Chuck Britton wrote:
> >  [0] For an airplane cruising at constant velocity - which force is
greater -
> >  a)      the drag produced by the air
> >  b)      the thrust produced the jet engine.
> >  c)      neither is greater - they are equal.
> >
> >  This is Newton's number ONE law. (which some pedagogs dismiss as a
> >  trivial special case of the Second Law.)
>
> Hmmmmm....
>
> The question does not look to me like a statement
> of Newton's first law ... nor even a corollary.
>
> Let's see what students would have to do, in order
> to connect question [0] with Newton's laws.  For
> starters, they would have to answer several background
> questions:
>    1) What's the definition of drag?
>    2) What's the definition of thrust?
>    3) What's the definition of cruise?
>    4) How is the thrust line oriented in typical aircraft?
>    5) How does the atmosphere affect flight?
>
> Pray tell, how is the student supposed to know these
> things?  I'd be willing to wager that most of the
> teachers who assign question [0] don't know the
> right answers to questions (1) through (5).
>
> By the way, under ordinary conditions, on average,
> the correct answer to question [0] is (a).  (Maybe
> thrust is not much greater than drag, but the question
> didn't ask about "much greater".)
>
> If you like answer (c), you'll have to re-word the
> question.
>
> ==============
>
> Questions like this give "story problems" a bad name.
> That's a shame, because story problems are super-
> important.  Real life feeds us a steady diet of story
> problems.
>
> In any case, if/when you assign story problems, make
> sure the students have -- or can obtain -- enough
> domain knowledge to be able to attack the problem.
>
> Story problems on homework, or during in-class
> discussion, are not so much of a problem, because
> students can hunt up additional domain knowledge if
> needed.  In contrast, story problems on closed-book
> multiple-choice tests are very hard to design, and
> very hard to grade.  If somebody answers (a) to
> question [0], how do you interpret this:
>   -- non-understanding of Newton's laws?
>   -- too little domain knowledge?
>   -- too much domain knowledge?
>
> Also:  If you're asking questions about the real world,
> don't ask narrow, brittle questions.  Suppose thrust
> is 0.1% greater than drag .... Do you put on your
> "practical purposes" hat and call them equal, or do
> you put on your "strict constructionist" hat and call
> them unequal?  I recommend neither of the above;  I
> recommend asking a more robust question, so that the
> responses don't require fussy interpretation.
>
> As a related thought:  Don't confuse story problems
> with laws of nature.  The laws of physics in particular
> are often very precise and/or idealized.  The real
> world is much messier.
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